Reference frequency oscillators play a significant role in the performance of modern integrated circuit devices and systems. With the development of integrated low-loss (e.g., <1 kΩ) microelectromechanical resonators, circuit designers are able to develop microelectromechanical oscillators to deliver highly-stable and low-jitter clock signals with smaller form-factor and lower power than oscillators using quartz crystals.
Despite their short-term and long-term stability, microelectromechanical oscillators may suffer from inferior frequency accuracy compared to quartz crystals, both in terms of temperature stability and manufacturing tolerance. For example, the relatively large temperature coefficient of frequency (TCF) of many microelectromechanical oscillators may cause significant frequency drift over a commercial temperature range. This relatively high level of frequency drift may make microelectromechanical resonators unacceptable for many applications, including those requiring ±50 ppm of accuracy. To address this potential limitation associated with microelectromechanical resonators, temperature compensation techniques have been developed. Some of these temperature compensation techniques, which include electrical compensation and material compensation, are disclosed in articles by K. Sundaresan et al., entitled “A Low Phase Noise 100 MHz Silicon BAW Reference Oscillator,” Proceedings of the Custom Integrated Circuits Conference, pp. 841-844, Sep. 10-13 (2006); H. M. Lavasani et al., entitled “Low Phase-Noise UHF Thin-Film Piezoelectric-on-Substrate LBAR Oscillators,” Proc. IEEE MEMS, pp. 1012-1015, January (2008); G. Ho et al., entitled “Temperature Compensated IBAR Reference Oscillators,” Proc. IEEE-ASME MEMS 2006, pp. 910-913, Jan. 22-26, 2006; and H. M. Lavasani et al., “A 145 MHz Low Phase-Noise Capacitive Silicon Micromechanical Oscillator,” IEEE IEDM, pp. 675-678, December (2008). The disclosures of these articles are hereby incorporated herein by reference.
As these articles describe, frequency tuning in microelectromechanical oscillators can be achieved by varying the resonance frequency of the microelectromechanical resonator and/or introducing additional phase shift in an oscillation loop. The continuous tuning of resonance frequency can be achieved by modifying the acoustic properties of the resonating structure by changing the electrical and/or mechanical stiffness of the resonating portion of the oscillator. Unfortunately, techniques for modifying acoustic properties by electrostatic and thermal tuning typically require relatively large DC voltages and increase power consumption. The absence of a polarization voltage also makes these techniques impractical for resonators requiring piezoelectric transduction.
Techniques to provide additional phase shift in the oscillation loop typically include using tunable/variable capacitors placed in parallel (“parallel tuning”) or series (“series tuning”) with the resonator. Parallel tuning usually changes the feedthrough capacitance to thereby cause a shift in the anti-resonance of the resonating element. The shift in anti-resonance will indirectly impact the resonance frequency, but the tuning range is mainly limited to the difference between the resonance (when the feedthrough is completely cancelled) and the anti-resonance frequency.
In contrast, series tuning provides the possibility of a theoretically unlimited tuning range. Thus, as illustrated by FIG. 1, in a laterally-vibrating microelectromechanical resonator, which may be modeled as a series RLC tank circuit 18 with relatively large parasitic shunt capacitors 15a, 15b (e.g., Cp≈2 pF), series tuning can involve placing a tuning network 10 in series with the resonator as the most efficient way to change resonant frequency. This tuning network 10 is illustrated as including a transimpedance amplifier 12 with tunable gain (provided by RF and CTUNE) and a voltage amplifier 14, which may drive an off-chip buffer 16. Unfortunately, the presence of the relatively large parasitic shunt capacitances may significantly reduce the tuning range to a level below what is necessary for adequate temperature compensation.